1. Field of the Invention
This invention pertains to novel compositions of matter and methods of using same in fracturing a subterranean formation penetrated by a wellbore.
2. Description of the Prior Art
Hydraulic fracturing is a term that has been applied to a variety of methods used to stimulate the production of fluids (e.g. oil, natural gas, brines, etc.) from subterranean formations. In hydraulic fracturing, a fracturing fluid is injected down a wellbore and against the face of the formation at a pressure and flow rate at least sufficient to overcome the overburden pressure and to initiate and/or extend a fracture(s) into the formation. The fracturing fluid usually carries a proppant (e.g. 20-40 mesh sand, bauxite, glass beads, etc.) into a fracture which keeps the formation from closing back down upon itself when the pressure is released. The proppant-filled fractures provide permeable channels through which the formation fluids can flow to the wellbore and thereafter be withdrawn. Hydraulic fracturing has been used for many years as a stimulation technique and extensive work has been done to solve problems present at each stage of the process. For example, the fracturing fluid is exposed to high temperatures and/or high pump rates and shear which can cause the fluids to degrade and to prematurely "drop" the proppant before the fracturing operation is completed. Considerable effort has, therefore, been spent trying to design fluids that will satisfactorily meet these rigorous conditions.
A wide variety of fluids has been developed, but most of the fracturing fluids used today are aqueous-based liquids which have been either gelled or foamed.
Aqueous gels are usually prepared by blending a polymeric gelling agent with an aqueous medium. Most frequently, the polymeric gelling agent of choice is a solvatable polysaccharide. These solvatable polysaccharides form a known class of compounds which include a variety of natural gums as well as certain cellulosic derivatives which have been rendered hydratable by virtue of hydrophilic substituents chemically attached to the cellulose backbone. The solvatable polysaccharides therefore include galactomannan gums, glycomannan gums, cellulose derivatives, and the like.
The solvatable polysaccharides have a remarkable capacity to thicken aqueous liquids. Even small amounts are sufficient to increase the viscosity of such aqueous liquids from 10 to 100 times or more. In some instances, the thickened aqueous liquid has sufficient viscosity to carry the proppant during the course of the fracturing process and represents a satisfactory fracturing fluid. In other instances, however, it is necesssary to crosslink the polysaccharide in order to form a gel having sufficient strength and viscosity to carry the proppant. A variety of crosslinkers have been developed to achieve this result at different pH ranges.
The borate ion has been used extensively as a crosslinking agent for hydrated guar gums and other galactomannans to form aqueous gels used in fracturing and other areas. For example, Kern described a crosslinked system in U.S. Pat. No. 3,059,909 which was used extensively in the oil and gas industry as a fracturing fluid. A fracturing process which comprised crosslinking, guar-containing compositions on-the-fly with a borate crosslinker was described by Free in U.S. Pat. No. 3,974,077. The borate-crosslinked systems require a basic pH (e.g. 8.5 to 10) for crosslinking to occur.
Other crosslinking agents were developed using certain transition metals. Chrisp described certain of these crosslinked systems in U.S. Pat. No. 3,202,556 and U.S. Pat. No. 3,301,723. In U.S. Pat. No. 3,202,556, aqueous solutions of galactomannan gums were crosslinked at a pH of from about 5 to 13 with antimony or bismuth crosslinkers. In U.S. Pat. Nos. 3,301,723 Chrisp described the use of certain titanium, zirconium, and other transition metals as crosslinking agents for galactomannan gums at a pH also in the range from about 6 to about 13. In both Chrisp patents, a basic pH was used to prepare crosslinked materials having utility in the explosive industry.
Another patent which described the use of titanium crosslinkers for solvatable polysaccharides was Tiner et al. (U.S. Pat. No. 3,888,312). The crosslinked gels formed by Tiner were said to be useful as fracturing fluids. The use of such crosslinked gels was alleged to overcome the high friction loss experienced during the pumping of many high viscosity fracturing fluids previously known. This observation corroborated the disclosure by Chrisp in U.S. patent No. 3,301,723 at column 10 that crosslinked gels formed using titanium, chromium, iron, and zirconium crosslinkers had a high surface tension (i.e. stickiness and tackiness are absent), ready workability and other desirable physical characteristics.
A class of thickeners was recently described by Syamalarao Evani in U.S. Pat. application Ser. No. 232,327, filed Feb. 6, 1981, now U.S. Pat. No. 4,432,881, entitled "Water-Dispersible Hydrophobic Thickening Agent"; the disclosure of which is incorporated herein by reference, Evani alleged broadly that such thickeners would be useful in a variety of fluids, including fracturing fluids.
Reference is made to the "Handbook of Water-Soluble Gums and Resins" by Robert L. Davidson, Editor, as published by McGraw-Hill, Inc. (1980) for an excellent treatise on water soluble polymers which includes a discussion on hydratable (or solvatable) polysaccharides. Reference is also made to "Hydraulic Fracturing" by G. C. Howard and C. R. Fast, Monograph Volume 2, Henry L. Doherty Series, published by the Society of Petroleum Engineers (1970) which is an excellent introduction to the subject of hydraulic fracturing, even, though it is now somewhat dated.